Garage door systems, such as shown in U.S. Pat. Nos. 5,803,149 and 6,326,751, include a garage door that is normally shifted between a substantially vertical orientation, where the door is in a closed position, and a substantially horizontal position, where the door is in an open position. Jack shaft operators as disclosed in the '149 patent are available that employ a spring-loaded drive shaft to assist in controlled shifting of the heavy weight of the door as it is moved between its horizontal open and vertical closed positions along a guide track as by application of a counterbalancing force thereto. For lifting the door open, a pull cable connected near the bottom of the door is spooled on a drum mounted to the rotating shaft.
Garage door systems have been developed that also use an upper cable operatively connected adjacent the top of the door to pull the garage door from the open position to the closed position. The upper cable is tensioned with an extension spring, such as disclosed in the aforementioned patents. The '751 patent also shows a torsion spring that exerts a torsional or rotational force on links that are pivotally connected in order to tension the cable. Such a torsion spring and link arrangement introduces undesirable complexities and pivot points that can quickly wear and fail with repeated cycling and especially over prolonged periods of garage door operation.
During winding and unwinding of the cables from the drum or drums, the cables are more likely to spool onto the drums improperly or actually fall off of the drums, also known as cable throw, unless properly tensioned. In particular, the cable not bearing the majority of the load tends to come off of its drum unless properly tensioned. For example, when the door is nearly to its closed position, the majority of the door's weight is supported by the lower cable, thus reducing the tension in the upper cable which, unless proper tension is applied, results in cable throw. Cable throw causes the improper winding and/or unwinding of the cable from the drum, resulting in the malfunction of the garage door system in terms of properly opening and closing as is desired.
The use of extension or coil springs to tension upper cables of garage door systems is problematic from a security standpoint. More specifically, extension springs are attached between the upper cable and the door. Generally, there is a pivotal bracket arm attached adjacent the upper end of the door at one end and to a roller at its other end with the spring operatively attached between the arm and cable. Accordingly, with the door closed, the spring allows an intruder to exert an upward lifting force on the door to push the roller in the guide track with the spring deflecting or stretching, thus raising the door despite lack of rotation of the drive shaft and drum on which the upper cable is spooled. In other words, the intruder can lift the door by way of spring deflection, even though the length of the upper cable between the drum and spring does not increase. The intruder usually will be able to lift the door by deflection of the spring by a vertical amount sufficient so that they can gain access to the interior of the garage by fitting under the door, e.g., by lifting the door by a height off the ground large enough for the intruder to pass through. Further, if the yield strength of the spring is exceeded, the overflexed spring may not be able to exert the same tensioning force on the cable and generally will see its usable spring life cycles reduced. In some instances an intruder may stretch the spring so that the spring breaks, thereby allowing the garage door to be lifted completely up.
A further complication in designing drive systems comes from the use of multi-panel doors that travel curved paths as these doors move between open and closed positions. As the panels pivot relative to adjacent panels during travel along the curved path, the respective distances traveled by between the top end and the bottom end of the door are not the same for a given elevation of the door. Since the upper and lower cables are attached to these ends of the garage door, the length of travel required of the upper cable also varies relative to the length of travel required of the lower cable as the door is raised and lowered. The variance in the travel distance of the cables can cause fluctuations in the tension in the cables, which can result in the build up of slack and thus cable throw.